In acoustic welding, such as ultrasonic welding, two parts to be joined (typically thermoplastic parts) are placed directly below an ultrasonic horn. In plunge welding, the horn plunges (travels toward the parts) and transmits ultrasonic vibrations into the top part. The vibrations travel through the top part to the interface of the two parts. Here, the vibrational energy is converted to heat due to intermolecular friction that melts and fuses the two parts. When the vibrations stop, the two parts solidify under force, producing a weld at the joining surface.
Continuous ultrasonic welding is typically used for sealing fabrics, films, and other parts. In continuous welding, typically the ultrasonic horn is stationary and the part is moved beneath it. Scan welding is a type of continuous welding in which the part moves. The plastic part is scanned beneath one or more stationary horns. In transverse welding, the part is stationary while the horn moves over it.
The horn is an acoustical tool made of, for example, aluminum, titanium, or sintered steel that transfers the mechanical vibratory energy to the part. Horn displacement or amplitude is the peak-to-peak movement of the horn face. The ratio of horn output amplitude to the horn input amplitude is termed gain. Gain is a function of the ratio of the mass of the horn at the vibration input and output sections. Generally, in horns, the direction of amplitude at the welding surface of the horn is coincident with the direction of the applied mechanical vibrations.
A rotary acoustic horn, like all horns, imparts energy at a selected wavelength, frequency, and amplitude. The rotary horn includes a shaft with input and output ends, and a welding portion mounted on and coaxial with the output end. The diameter of the welding portion is greater than the diameter of the shaft. The welding portion has a cylindrical weld face having a diameter that expands and contracts with the application of acoustic energy. Typically, a rotary horn is cylindrical and rotates around a longitudinal axis. The input vibration is in the axial direction and the output vibration is in the radial direction. The horn and anvil are close to each other, and the anvil can rotate in the opposite direction to the horn. The part to be bonded passes between these cylindrical surfaces at a linear velocity which equals the tangential velocity of these cylindrical surfaces. Matching the tangential velocities of the horn and the anvil with the linear velocity of the material is intended to minimize the drag between the horn and the material. The excitation in the axial direction is similar to that in conventional plunge welding.
U.S. Pat. No. 5,096,532 describes two classes of rotary horn. The patent compares a commercially available full wavelength rotary horn, manufactured by Mecasonic-KLN, Inc. of Fullerton, Calif. (Mecasonic horn) and a half wavelength rotary horn described in the '532 patent.
U.S. Pat. No. 5,707,483 discloses another type of rotary acoustic horn with undercuts.
There are typically two methods of mounting any ultrasonic horn, nodal and non-nodal mounting. A node is a portion of the horn that is not moving in one or more directions. With a nodal mount the horn can be held or grasped rigidly. Non-nodal mounts require some flexible elements because the horn surface is moving (vibrating). Because of the difficulties of handling the vibrations, non-nodal mounts are typically not used in the industry.
Nodal mounts typically have a flange machined at a node, shown in FIG. 1, or a series of set-screws positioned radially around the node shown in FIG. 2. U.S. Pat. No. 4,647,336 discloses a reparable nodal flange mount such as that of FIG. 1. In this design the flange at the nodal point of a booster has an O-ring above and below it. A two piece support collar clamps against the O-rings to support the assembly. (This is shown in FIG. 4 of the '336 patent). U.S. Pat. No. 4,995,938 discloses using the nodal flanges as pistons of an air or hydraulic cylinder. In this system, the method of supporting the booster and the method of applying the required welding force are combined, as shown in FIG. 1 of the '938 patent).
U.S. Pat. No. 5,486,733 discloses a nodal mount inside the converter. A machined ring is sandwiched between the piezoelectric crystals that are driving the horn. U.S. Pat. No. 4,975,133 includes a set-screw nodal mount for an ultrasonic booster. This design is used for rotary shear welding operation. RD 21128 discloses a method where a potting material is used instead of O-rings on a nodal flange type mounting.
Anti-nodes are areas of maximum displacement of a horn or booster. Attaching mounting systems at these locations or at other non-nodal locations requires the mount to be designed to isolate the vibration from base of the device.
U.S. Pat. No. 3,752,380 discloses using a pair of leaf springs located at the non-nodes of a non-rotating bar horn. U.S. Pat. No. 3,863,826 discloses a sonic or ultrasonic apparatus that uses a leaf spring support to mount a converter to a stationary support. These springs isolate the vibration and allow vertical motion with an air cylinder to accommodate the height of the parts being welded. This design is for a non-rotating horn. In both these patents, the leaf spring is sandwiched between the junction of the converter or booster and the horn. This interferes with the transmission of ultrasonic energy through the horn and limits the usefulness of rotary horns.
U.S. Pat. No. 3,955,740 discloses a non-nodal rotary horn mount which uses a solid metal diaphragm located in the junction between the booster and horn. The diaphragm is statically stiff, having a static stiffness of about 1.35.times.10.sup.7 N/m (77,000 lb/in). Very high static loads are possible because the design uses a rotating tube to isolate this force from the bearings Also, in this design, the diaphragm is designed to resonate at the frequency of the horn. The diaphragm is an ultrasonic element. Similarly, U.S. Pat. No. 4,884,334 discloses a statically stiff disk or finger support which resonates at the frequency of the horn and is an ultrasonic element.
U.S. Pat. No. 5,468,336 discloses a flexural spring support for a shear welding apparatus. This design uses tapered support beams that allow a plate to be vibrated side-to-side while remaining parallel to the fixed plate. This side-to-side motion slides the parts being welded past each other and welds them together. This design is for a shear welding operation, uses taper beams, and could not be practically adapted to a rotary horn mounting situation. U.S. Pat. No. 5,464,498 discloses another flexural support spring method. This design is machined from a single piece of material.
A commercially available non-nodal mount for horns, shown in FIG. 3, was manufactured by American Technology, Inc. (Amtech), Shelton, Conn., and is used for shear welding. This unit has a plate with slots machined radially from the outer diameter to create fingers. The outer edge of the finger plate is mounted with a clamp ring while the inner fingers are clamped between the end of the horn and the booster.